Acoustic device



April 1933- H. c. HARRISON 1,905,704

ACOUS TIC DEVICE Filed May 14, 1950 s Sheets$heet 1 W ml -'|"|l K I Q U7 I v/ u I /Nl EN7'O/? v /0 I hi C. HARP/SON Wm 7M ATTORNEY llllmml I MnApril 25, 1933. c, so 1,905,704

ACOUSTIC DEVICE Filed May 14, 1950 3 Sheets-Sheet 2 uvvavrop H. C.HARP/SON am? 12 M ATTOP/VE) Patented Apr. 25, 1933 UNITED .STATESPATENT. OFFICE HENRY C. HARRISON, OF, PORT WASHINGTON, NEW YORK,ASSIGNOR TO BELL TELE- PHONE LABORATORIFS, INCORPORATED, OF NEW YORK, N.Y., A CORPORATION OF NEW YORK Application filed May 14,

This invention relates to acoustic devices and particularly tosound'reproducers of the direct acting ty e. i

The vobject 0 this invention is to radiate sound energy withsubstantially uniform efficiency over at least a large portion of theentire frequency range of speech and music.

One feature of this invention resides in the sound radiator employed totranslate the vibrations impressed thereon into sound disturbances.

Another feature of this invention'resides in the driving means employedto impart acoustic vibrations to said sound radiator.

Still another feature resides in the magnetic structure forming a partof said driving means.

In accordance with this invention there is provided a direct actingsound radiator com prising a plurality of tensioned members to which issecured some thin material to prevent acoustic short circuit between 0posite surfaces thereof and drivin means t erefor comprising a currentcarrying member of a material having a high ratio of conductivity tomass, said stri being secured to and across one face thereo andintermediate its ends.

This current carrying member is disposed in an elongated magneticair-gap formed by a pair of'parallel pole pieces which are ele-v mentsin a magnetic structure which allows a maximum and uniform distributionof magnetic flux with the use of a comparatively small amount of copperin the energizing coils of the magnet.

A more complete understanding of this inventionwill be obtained byreference to the ap ended drawings where n:

ig. 1 shows fragmentarily an embodiment of this invention incorporatingthe feaacousrrc DEVICE 1930. serial No. 452,154.

showing in detail the means employed for securing the extremities of thetensioned elements employed in the sound radiator;

Fig. 5-A shows a variation in the .tensioned member securing means shownin views of a portion of the magnet structures shown in Figs. land 2;and i Fig. 10 is a section of the magnetic structure shown in Fig. 1along the line 10-10.

Referring now to the drawings, Figs. 1 and 2 show a sound reproducerincorporating the features of this invention. I There is provided aframe whichsupports a diaphragm 61, a magnet structure 16 and atransformer 14 which latter is provided with input leads 15. This framecomprises vertical supports 1 which may be of iron pipe or othersuitable material and horizontal'channel members 11 which extend betweenthe extremities of the vertical supports and may be secured thereto inany desired manner preferably by welding. The frame. is strengthened byangle irons 3 extendin between'the vertical supportsand by ang e irons 4which extend angularly between said members 3 and supports 1. Members 3and 4-are welded preferabl to one another and to the supports. To givethe frame greaterrigidity, bracing members 2, which may be of iron pipe,are secured at their extremities to the vertical supports 1, preferably,by welding, and are maintained spaced therefrom by spacer members 7 and8. rThe spacer members 8 comprise studs secured *to the bracing member 2and the support 1 and are coupled together by a turn buckle element 9.

' This sound reproducer may be placed on any fiat surface resting oneither its lon er or shorter dimension, or ma be supporte from aceiling, wall or frame y means of the eye bolts 10 which are secured tochannel members 20, in turn secured to the supports 1 by pat bolts 44and nuts 45. Intermediate the extremities of the frame, and in operativerelation to the diaphragm 61, is the magnet structure 16 which issecured at its opposite ends to the supports 1 preferably by welding andsupported by the members 5 and 6 which extend angularly between thelower portion thereof and the supports 1. The magnet structure will bemore fully described hereinafter with reference to Figs. 6, 8, 9 and 10.Figs. 1 and 2 show, further, the energizing coils 17 therefor and theterminals 19 for the strip member 40 (Fig. 3) whose movements arecommunicated to the diaphragm and cause it to vibrate and radiate sound.Connected to the terminal blocks 19 are conductors 18 of highlconductive material and of lar e cross section which provide the returnTi to the transformer 14 for the currents passing therefrom to theconducting strip 40. Conductors 18 may be of aluminum or ofcopper or ofany other material having preferably a low impedance and ma besecuredconveniently to, but conductive y insulatedfrom, a portion of themagnet structure. As shown in Fig. 10, this may be accomplished atdefinite intervals along its length by spacin the conductor 18 from theportion 27 of tie magnet structure by a washer 42 of insulating materialand securing it against the latter by screw 43 wh ch threads into aT-sectioned block of insulation 41' which fits into a recess in'theportion 27. The diaphragm 61 comprises a plurality of parallel tensionedmembers 13 spaced very short distances apart and extending between theextremities of the frame, and a thin material 12 preferably, imperforateto air, which is secured to said tensioned members to prevent acousticshort circuit between the opdposite surfaces thereof. The length of theiaphragm is preferably greater than twice its width and also greaterpreferably than onehalf the wave len h of the lowest frequency to beradiated. u'ch a diaphragm has been found highly satisfactory as a soundradiator which ma be tensioned to a desired degree in the direction ofits lon dimension only, if desired, and having su stantially no tenssiontherein in a direction transverse to its len h. The diaphragm isreferably tensione to such a degree that t e sound vibrations arepropagated therein at a velocity greater than one-quarter that of soundin 1113 {It may be desirable to introduce a'slig'ht lateral tension inthe diaphragm to maintain the desired velocity of propagation in thediaphragm and to counteract the effect of the mass of air into which itis working which has a tendency to reduce the attenuation. Such adiaphra is disclosed in R. L. Wegel Patent 1,87 0,2 1, patented August9, 1932. As here employed, tensioned members 13 may be wires ofduralumin and the thin material secured to them may be of paper.

A means for securing these wires in spaced relation and maintaining themin that condition is shown in Figs. 4 and- Secured by bolts 50 to thechannel member 11 extending between the supports 1 is a member 49 whichis preferably of a material similar to that employed for the tensionedelements 13. This member 49 has parallel saw cuts 52 and at right anglesthereto a series of saw cuts in which the extremities of the elements 13are positioned and clamped against movement by forcing the portions 53between the vertical saw cuts about the portions of the tensionedelements in each saw cut. It will be understood that one of thesemembers is employed at each extr'emitypf the tensioned elements and thatone extremit of each wire may be initially secured in sucli means, thenthe other extremity subjected to the tensioning force necessary toproduce the desired condition of strain therein and thereafter clampedin a like securing means. As a protection to the wire securin member 49,an elongated-member 46 is positioned over itiand maintained in place bybolts 47 which roject through the members 46 and 49 an the channel crossmember ll; Fig. 5 shows also a strip 51 of thin flexible material, .suchas leather, which is secured to the support 1 and to the lateral edge ofthe diaphragm. Such a flexible strip is employed alon' each lateral edge'of the diaphragm exten ing in the direction of its longer dimension to,prevent acoustic short circuit between 0 posing surfaces of the soundradiator in t e area between the lateral edges thereof and the verticalsupports 1 of the frame.

Substantially the result obtained with the tensioned element holdingmeans of Fig. 5 may be secured by the modification thereof shown in Fig.5-A. Cross cuts 52 are omitted and continuous parallel vertical slots 62are provided in which the'extremities of the elements 13 .are placed.They are secured therein by causing the material between the slots 62 tooverlap the elements 13therein as shown. This may be accomplished byusing a punching tool and striking the member 49 at the staggered points63 with suflicient force to cause the desired displacement of thematerial between said slots thereby clamping the wires therein.

The diaphragm or sound radiator comprising tensioned elements 13 andthin material secured thereto is driven by a current conductin stripwhich is secured intermediate the en s thereof to and across the face ofthe diaphragm at substantially right angles thereto. This strip is of amaterial having a high ratio of conductivity to mass and is disposed inan elongated magnetic airap ormed by the parallel pole pieces of an eeetromagnetically energized magnet structure 7 Referring to Fig. 3, aportion of this contrial havin non-integral feet ortions which ductingstrip is shown and is, preferably, of aluminum or copper. As shown it isthin,

'flat and ribbon-like in structure, altho a plurality of such stripsforming a portion of the winding of an elongated coil could be employed.It is provided with a pluralit of feet 54 eachof which is secured to one0 the tensioned elements 13 providing what is virtually an individualdrive for eachltensioned element. Near each extremity of this elongatedstrip is a portion 55 which is shredded or slotted to give itflexibility in that region. This flexibility could also be obtained byhaving that portion of the strip of thin" foil but of appropriate crosssection. Each end of the strip may be secured directly to the terminalblock 19 by soldering it thereto, if desired, or, as shown, by means ofascrew 39.

Terminal block 19 which maybe of a material such as brass is fastened tothe support 1 by a screw 57 which is surrounded by a sleeve 58 ofinsulating material to avoid a metallic circuit between the terminalblock and support. The terminal block is electrically insulatedtherefrom further by a plate 'of insulating material 59, such as hardrubber.

Guard member 56 secured'to the terminal in section at'all pointstherealong or that it] may consist of aribbon-like strip ofmatearesecured-to. one edge thereo and providing means for positioningthe conductor wholly within the field of the'magnet structui'e and.

' for securing it to the diaphragm. As a further modification, feetportions 54 may be omitted altogether and the conductor secured directlyalong one edge thereof to and across the face of the diaphragm,substantially at ri ht angles thereto.

. ig. 7 is a schematic of the circuit for the electrical impulses whichare to be reproduced by the sound reproducer incorporating the featuresof this invention; The conducting strip 40 is shown connected at itsextremities to the terminal blocks 19 as are also the return conductors18 to the secondary of the transformer 14 whose primary is shownconnected in the output circuit of the last stage 60 of anamplifier. Thereturn conductors 18 are of 'aymaterial of high conduc-' tivity,forinstance,--.of copper, and are of large cross section'to reduce to aminimum dissipation of energy therein. Preferably the impedance of thestrip 40 is substantially equal to the combined impedances of thesecondary of the transformer 14 and the low impedance return conductors18 which are shown in Figs. 1, 3 and 7 as connected to the terminalblocks 19. It will be apparent that a ne ligible percentage only of thecurrents flowing through the strip 40 will pass through the feetportions 54 and that these portions may be connected directly to thetensioned elements 13. In the normal position of the strip 40 in theelongated air-gap of the magnetic structure, these feet portions neednot be therewithin. This provides the necessary clearance between thediaphragm and the magnet structure without any appreciable portion ofthe currents passing through the strip 40 traversing the portion thereofwithout the magnetic air-gap. If the feet portions 54 were ofnon-conductive material, the current fiow in the strip or conductor 40would be whollyin that portion thereof within the magnetic air-gap. Thenormal position of the conducting portion 40 is shown clearly in Fig.10.

vThe detailed description above has been .with reference to a diaphragmcomprising a plurality of tensioned elements closely s aced together andwith a thin material imperfbrate to air secured thereto to preventacoustic short circuit between opposite surfaces thereof. Although thissound radiator is preferred, it will be understood that a uniformlytensioned sheet diaphragm such as is shown in my Patent 1,726,105,issued August 27, 1929, may be employed or a transversely corrugatedsheet diaphragm sucli as is disclosed in E. H. Smythe Patent. 1,821,586,issued Sept. 1, 1931. Furthermore instead of using a diaphragm of asingle large sheet as disclosed in the patents referred to above, onemade up of a plurality of elongated strips of uniformly tensioneddiaphragm material may be employed. y

The magnet structure forming a feature of this invention will now bedescribed with ref erence to Figs. 1, 6, 8, 9 and 10. As shown in Fig.1, ma net structure 16 extends between the vertica supports 1. Itprovides an elongated air-gap 64 in which is positioned a con- Iuctingstrip 40 secured across the face of the diaphragm 61. An elon atedpermanent magnet with oppositely disposed pole portions might beemployed to provide the desired uni-directional magnetic field across,said airap but for the purpose ofthis disclosure e ectromagnetic meansare preferred.

It is understood pf course that a conductor disposed in auni-directional magnetic field and carrying an electric current will beacted upon by forces whose direction and magnitude dend upon thecharacter and amplitude o the currents flowing therein, the stren h ofthe magnetic field in which it is positioned, and the direction in whichthe latter is acting. Further, to have the conductor actedupon along itslength with the same force at all points thereon with a given currentflowing therethrough, the strength of the uni-directional magnetic fieldshould be uniform at all points there along. This condition has beenattained with the structure now to be described, said structure beingcompact and requiring a minimum of copper in its energizing coils.

Figs. 6, 8 and 9 are exploded, plan and rear elevation views of slightlymore than onehalf of the magnet structure, it being understood that thestructure is symmetrical in outline. A metallic plate 24 of a magneticmaterial, such as soft iron, is secured to an angularly disposedmetallic plate 27, which may be of soft iron also, which forms one olepiece of the electromagnet. The memr 27 has a tapered edge portion 65convergin to form a long narrow ed 66, so that w en magnetic flux isroduce in said pole piece, it is concentrate in an elongated rectangulararea. To give rigidity to this assembly, blocks 34 and 28 are fastened,for

instance, by welding, to their inner surfaces.v Surface 35 of block 34is curved to fit the;

curvature of supports 1. An opening 67 is provided in strengtheninmember 28 for the conductor 18 which 15 secured to but electricallyinsulated from member 27. To

insure a uniform distribution of magnetic flux to the member 27, members23 of magnetic material and triangular in shape are ositioned on theunder surface of the memer 24 and may be secured thereto in any suitablewa fer instance by welding. Supported by t e member 27 are blocks 32whose upper portions 33 are of a non-magnetic material such as brass andare provided with tapped drillings 68 into which screws 38 may bethreaded. Supported by the member 24 are cores 31 of magnetic materialwhich are surrounded by energizing coils 17 which may be connected to asource of current when it is desired to magnetically energize the cores.

The core is secured to the triangular mem-' ber 23, preferably bywelding it thereto but a screw projecting through said member and intothe core would be satisfactory. The other pole of the electromagnetcomprises members 22 which are also triangular in shape. Each member 22has an opening'69' therein into which the core 31 projects and dowel ins30 with which the openin the V- aped members 21 register. embers 21 and22 are of magnetic material and may be fastened to one another by anysuitable means. A screw 37, passing through member 29 and threadin intothe core 31, and screws 38, passing through the member 22 and threadlninto the ortions 33 of the blocks 32, hol these mem rs firmly in place.The portion of the member 22 projecting out beyond this support by theblocks 32 is so shaped as to concentrate the magnetic flux which may bepresent therein and terminates in an elongated surface 71 opposite- Idisposed with reference to the surface 66 orming the other pole face ofthe electromagnet.

It will be apparent that the structure described permits of a uniformdistribution for the magnetic flux produced in the core 31 whenenergizing currents are flowing in coils 17 and rovides a long narrowair-gap within whic a current conducting strip may be placed. The widthof the air-gap may be varied by using different thicknesses of washers36 between the under surface of the member 22 and the upper portion 33of the block 32. It will be apparent, also, that, if desired, elements21 and 22 may be formed as an integral unit'as may, also, elements 23and 24.

There has been describeda sound re roducer of the electro-dynamic typein w ich the moving element comprises a conducting strip disposed in anelongated magnetic airgap and secured to and across the face of anelongated diaphragm which is in a condition of uniform tension and whichcomprises preferably a plurality of tensioned elements securely held attheir extremities. The conducting strip is referably provided with amultiplicity of eet portions, one secured, preferably, to each tensionedelement and is adapted for movement in accordance with the sounds to bereproduced, said movement resulting from motor action between thecurrent conducting strip and the uni-directional magnetic field in whichit is located; the movements being in the plane of the conductor. Thesemovements are communicated to. the diaphragm and sound energy isradiated therefrom to the atmosphere.

It is to be understood that the embodiment of this invention describedabove and shown on the appended' drawings is illustrative of theinvention and that its scope is to be limited only by the appendedclaims.

What is claimed is:

1. A sound reproducing device comprising an elongated direct actindiaphragm, means defining a magnetic air-gap and a current conductingmember disposed in said air-gap and secured to and across the face ofsaid diaphragm along its short dimension, said member having integralflexible portions near its extremities.

2. In combination, a direct acting diaphragm comprising a plurality ofelongated elements, means defining a magnetic air-gap, and a currentconducting member disposed in said air-gap and secured to saiddiaphragm, said member havin a plurality of projecting portions, each ofwhich engages with at least one of said elongated elements.

3. In combination, a direct acting diaphragm comprising a plurality ofelongated meta lic elements, air-gap, and a. current conducting memberdisposed in said air-gap and secured to said diaphragm, said memberhaving a plurality of projecting portions, each of which is attached toat least one of said elements.

4. In combination, a diaphragm having a length substantially greaterthan twice its means defining a magnetic width and more than one halfthe wavelength of the lowest frequency to be reproduced, eom n'ising aplurality of spaced elements and a thin nmterial secured thereto, saiddiaphragm being tensioned substantially along its length only, meansdefining a magnetic air-gap, and a current conducting member attached tosaid diaphragms spaced elements. said member being disposed in saidmagnetic air-gap.

5. In combination, a diaphragm comprising a plurality of spacedelongated elements, means for securing. said el ments at theirextremities. said meiins comprising a memher having depressions intowhich the extremities of said elements are placed and clamped therein bydisplacement of the material between said depressions, and means foractuating said diaphragm. 6. In a sound reproducing device, a frame, adiaphragm supported by said frame, and comprising a plurality ofmetallic elements to which is secured a thin material imperforate toair, thin flexible means intermediate the lateral edges of saiddiaphragm and frame to minimize acoustic short circuit between thesurfaces of said diaphragm, means for actuating said diaphragmcomprising a thin flat conductor secured to and across one surface ofsaid diaphragm intermediate its ends, said conductor having flexibleportions near its extren'iities, and a magnet structure providing anelongated magnetic air-gap in which a portion of said conductor isdisposed. 7. In eoi'nhination, a diaphragm and means for driving saiddiaphragm, said means comprising a conducting member secured to saiddiaphragm and a magnet structure including pole pieces disposed to forma magnetic airgap, each of said pole pieces being so shaped as to insurea uniform distribution of magnetic flux. at all points along saidair-gap one of said pole pieces comprising a triangular shaped portionand a V-shaped portion. in an acoustic device, a diaphragm comprising aplurality of elongated metallic elemeats. means defining amagnetic airgap, and a current conducting member disposed in. said air gap andsecured to said diaphragm, said member comprising a plurality ofserratioi'is each of which is attached to at least one of said elements.

9. In combination, a diaphragm and means for driving said diaphragm,said means comprising a conducting member secured to said diaphragm anda magnet structure including pole pieces disposed to form a magnetic airgap, one of said pole pieces comprising a triangular shaped portion anda V shaped portion. the apex of said V being farthest removed from theair gap, and the arms of the V extending toward the magnetic air gap. InWitness whereof, I hereunto subscribe my name this 12th day of May,1930. HENRY C. HARRISON.

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